1. Field of the Invention
The present invention relates to an optical information recording medium and a disk substrate used for it such that an information signal of high signal quality is recorded by irradiating a thin film of reversible phase change material formed on a substrate with a high energy beam such as a laser beam.
2. Description of the Related Art
There is known a technique that forms spiral or concentric concavo-convex tracks (called groove tracks hereafter) on the surfaces of an optically smooth double-sided disk substrate of resin or glass. There is also known a technique that constructs a recording medium using a disk having these groove tracks.
In a recording medium of this type, the precision of recording and reproduction is increased by retaining a light beam used for recording and reproduction on a predetermined position along an above groove track (e.g. in a groove or on a part between neighboring grooves called land). In general, if light is made incident on a recording medium having groove tracks, the light reflected from a concave part and the light reflected from a convex part have the optical phase difference twice as long as the depth of the concavo-convex surface. Therefore, if the depth of the groove tracks is set so that the two reflected rays weaken each other, then the returning light has a minimum intensity if it is reflected on a neighborhood of the boundary between the concave and convex parts, and the reflecting light increases as it is reflected on a place farther from the boundary. Therefore, by detecting the intensity of the returning light, the position of the light beam is detected, so that retaining (tracking) the light beam on a land or in a groove can be achieved. Servo technology including this tracking technique is described, for example, in Hikari disuku gijutsu handobukku published by Nikkei-Mcgraw.
A byproduct of using grooved tracks is thermal effects described in Japanese patent Heisei 2-10492 (U.S. Pat. No. 4,385,303). That is, since the cross section of the recording film is thinner in the slope part than in the bottom part of a groove or on a land, heat is harder to diffuse in the slope part. Consequently, the efficiency of the laser beam used for recording is increased.
As for the shapes of groove tracks, prior techniques have formed groove tracks having the same groove depth, the same groove width, and the same groove pitch all over the disk to obtain the same tracking signal and the same reflectance regardless of the locations of the groove tracks.
As a method of forming groove tracks on a resin substrate, the following method is known. First, the method spin-coats a smooth glass disk with ultraviolet-hardening resin as thick as the groove depth and cuts groove track information including an address signal by radiating an Ar laser beam. If the lands are to be made the recording part, two laser beams are used to record information such as addresses between grooves ie. on a land while cutting a groove. After developing this plate, the method removes the non-hardened parts by etching, sputters the plate with a nickel thin film, plates it with a thick nickel film, and removes from the glass disk the rest of its layers to obtain a metal stamper as a master plate, so that groove tracks are formed by injection-press molding using this master plate.
As a method of forming groove tracks on a metal plate or a glass plate, a method referred to the 2P method deposits ultraviolet-hardening resin on a substrate, and hardens the resin by irradiating ultraviolet light while pressing the surface with an above stamper. Another method called the sol-gel method is also known. These methods of making a disk substrate having groove tracks are described in detail in Hikari disuku, edited by Denki-joho-tsushin gakkai, Ohm, pp 47-48, Zoku.Wakariyasui hikari disuku, Optronics, pp 143-194, Hikari disuku gijutsu handobukku, Nikkei-Mcgraw, p 14-15, and Hikari-disuku-yo zairyo gijutsu, CMC Electronics series, pp 130-134, and etc.
Among known recording thin film materials formed on a surface of groove tracks are phase change recording materials such as Ge--Sb--Te and In--Sb--Te that apply reversible phase-changing between the amorphous and crystalline states and are used as recording thin films at heat mode. There are also magneto-optical recording materials that are composed of rare earth elements and transition elements and apply the detection of the spin of a vertically magnetized film using Kerr effects. There are also Te-based inorganic material films or organic pigment films in which pits are made for recording and reproduction. If these thin films are irradiated with a laser beam, then the physical and chemical characteristics of an irradiated part change to cause an optical change. Only one-time recording can be made in pit recording, but phase change material and magneto-optical material can make the above change reversibly, so that recorded information can be replaced. In rewritable recording, dielectric thin film layers are formed on both sides of the recording film to enhance repeatability, to reduce external influences, and to enhance recorded signals. There have been also reports on increased light absorption in the recording layer by adding a reflecting layer and using the reflecting layer as a heat sink layer as well. The material technology is described, for example, in Hikari-disuku-yo zairyo gijutsu, CMC Electronics series. Generally each layer is formed to have a uniform composition and film thickness throughout the substrate.
However, as described in the following, prior optical information recording media described above have different recording conditions between an inner part and an outer part of the disk, so that it is difficult to realize equivalent recording characteristics in both inner and outer parts. For a disk-shaped optical information recording medium such as an optical disk to have large memory capacity, it is necessary to use an area of the disk as large as possible from its outer to inner part. However, memory requiring high-speed random access such as a disk for storing data files is driven by the CAV method (the driving system that keeps the revolution of disks constant). Therefore, the relative linear velocity between the optical head and the disk medium (called simply linear velocity hereafter) proportionately increases with the radial location of recording. If the size d of a light beam is constant, the heating time .tau. is uniquely determined by the linear velocity v as .tau.=d/v. Therefore, if the difference of radial locations between an inner part and an outer part is made great to enlarge the recording area, obtaining equivalent recording characteristics in both the inner and outer parts becomes difficult, since the irradiation time changes.
The above problem is described in more detail in the following. In order to change a recording film of a phase change material from the crystalline phase into the amorphous phase, a crystalline part is locally heated at a temperature beyond the melting point and rapidly cooled to freeze its random atomic arrangement. Therefore, heat must be diffused at a proper rate, after the recording film is heated beyond its melting point (See, e.g. N. Yamada et al. Jpn. J. Appl. Phys. Suppl. 26-4, 61(1987)). Since heat tends to be accumulated in an inner part of a disk, where irradiation time is comparatively long, special care must be taken to ensure that necessary cooling conditions for amorphizing be satisfied in an inner part. However, if a rapidly cooling structure is taken to satisfy amorphizing conditions for an inner part with very low linear velocity (e.g. a few m/sec), then temperature is hard to increase in an outer part with linear velocity several times greater, and recording power becomes insufficient.
Conversely, in order to change the recording film from the amorphous phase into the crystalline phase, an amorphous part is kept heated at a temperature beyond its crystallizing temperature (below its melting point) to recover the order of an atomic arrangement. Therefore, the temperature has to be kept at that temperature for more than a predetermined time period. Even the fastest phase change material obtained so far requires at least a few 10 ns of heating time for crystallizing. Therefore, if the linear velocity becomes a few 10 m/sec, then the irradiation time becomes equivalent to the heating time for crystallizing, so that crystallization is hard to achieve. Therefore, a structure reducing thermal diffusion (gradually cooling structure) has to be taken to prolong the time for keeping at beyond the crystallizing temperature. However, in that structure, an inner part with the linear velocity a few 1/10 times as fast as an outer part (e.g. a few m/s) does not satisfy the rapid cooling conditions. That is, in some irradiated part, a liquid part is recrystallized (ie. an amorphous part becomes smaller), so that recording signal amplitude decreases. Therefore, if the difference of radial locations between an inner part and an outer part is sufficiently large, then recording performance decreases either in an inner part or in an outer part. In other words, a recording medium allowing a sufficiently large difference of linear velocities has been hard to make.
In order to compensate the difference of recording characteristics between an inner and outer parts, prior methods of changing the thickness and composition of a recording film or a heat sink layer between an inner and outer parts (thickening in outer parts) were proposed. However, changing thickness and composition depending on locations has a problem with respect to reproducibility. In particular, in a recording medium having multilayer structure, which is already in practice, a few percent of change in film thickness brings a large difference of recording characteristics, so that these methods are not appropriate for mass production.
The Japanese patents Heisei 2-33740 and Heisei 2-94045 proposed magneto-optical recording disks driven by a CAV method such that groove width is varied depending on the linear velocity changing with radial track locations, so that recording power and erasing power are controlled to be constant.
However, the above proposals are limited to magneto-optical recording type, and if groove width is varied in inverse proportion to the linear velocity, then the rate of change in groove width becomes too large to be practical. Moreover, if the power is constant, then tracking performance in an outer part decreases, so that groove width in an outer part has to be made sufficiently large to maintain tracking performance, and consequently the improvement of recording density becomes hard to achieve.